Lateral and Vertical Distributions and Textural Features of Filamentous Bacterial (Beggiatoa sp.) Mats in Santa Barbara Basin, California

GRANT, CHRISTIAN W., Chevron Oil Field Research Company, La Habra, CA

Sixty bottom-water photographs and twenty-five box cores taken from Santa Barbara basin, California, were used to assess lateral and vertical distributions and textural features of sulfur-oxiding filamentous bacterial mats in surface sediments. Box cores span environments ranging from anaerobic (total dissolved O(2) in bottom water < 0.1 ml/L) to aerobic (O(2) 2 > 1.0 ml/L). Modem-day sulfur-oxidizing bacterial mats have been proposed as analogs to fossil bacterial mats identified in organic-rich source rocks such as the Monterey Formation.

Within a 225 sq km area (designated here as the "mat zone") of the anaerobic environment, two mat textures are recognized. The first consists of discontinuous white patches (4-20 cm in diameter) mottling a featureless gray background. The second texture shows a continuous crenulated mat surface. The mottled texture was recognized in eight photographs and the crenulated texture was observed in one photograph. The "mat zone" is coincident with the area of highest measured total organic carbon (TOC) in surface sediments.

Epifluorescent microscopy was used to identify filament biomass in surface sediments removed from box cores. Surface sediments within a region approximately 200 sq km in the anaerobic "mat zone" have sulfur-oxidizing filaments present, supporting observations of bottom-water photographs. Surface sediments from the deepest portion of the basin have a very high filament biomass, forming an area about 40 sq km in extent within the "mat zone." Outside of the 225 sq km "mat zone," bacterial filaments in surface sediments are absent, TOCs diminish, macrofauna increase in abundance, and bioturbation of sediments is pervasive. Analysis of sediments downcore shows filaments to be increasingly degraded below I cm depth, becoming nonexistent at 4 cm.

Scanning electron microscopy, epifluorescent- and plane-polarized light microscopy, and thin section analyses were conducted on sediments from cores to document sediment texture and composition of the different environments. Clay-rich sediments from the aerobic and dysaerobic zone are unconsolidated, lack bacterial filaments, and have surface porosities of approximately 84%, decreasing to 75% at 15 cm core depth. In contrast, anaerobic sediments with mats present contain abundant filaments that bind together diatom frustules, detrital clay and silt, fecal pellets, and amorphous organic matter into a semicohesive mat. Surface porosities within a mat are as high as 95%, decreasing to 85% at a depth of 50 cm.

X-radiography and thin-section microscopy reveal the presence of well-defined seasonal laminations (varves) in anaerobic sediments. Mats, primarily formed during the summer and early autumn months, are evident in thin section as highly porous, crinkly laminae alternating with brown laminae composed of detrital silts and clays deposited during the winter and early spring months.

Biomarkers unique to the mats are currently being analyzed using gas chromatography-mass spectrometry to chemically detect the presence of sulfur-oxidizing bacteria in sediments throughout the basin to substantiate observations discussed above. In addition, biomarker analyses will be used to identify possible precursors to geolipids found in analogous ancient source rocks, such as the Monterey Formation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)